Finally got around to reading Realms of Power: Magic, and decided to share some thoughts on it.
I found the best feature of the work to be the tiny nuggets of Mythic flavor connected to the various ideas. I really liked how you can Enrich an Object of Virtue (or Beast of Virtue and so on) to get Virtues, but at the cost of Warping and with zero Penetration so that it isn't really increasing the magi's power level but is providing lots of flavor and hedge-magic uses. Likewise the small effects of Aligned Auras were very nice. These tiny things really do, I feel, bring out the Mythic in Mythic Europe.
I also liked many of the individual entries (magical creatures and so on). Just going from memory, the dragon-wannabe drake, the magical city of birds, the lore on primordial giants, lost children - all made an impression. Lots of the other entries were very nice too, like the vis sources.
I found the idea of tethers mediocre. It was nice to give an idea on what a powerful aura would involve, but not so nice in being overly mechanical. Likewise the idea of the aura changing was good, but the implementation somewhat mechanical - I found myself much more inspired by the organic nature of the Forest Spirit's dissolution and growth in Guardians of the Forest. I certainly didn't like the spell guidelines for manipulating auras, which made things too easy for magi (I'd just prefer they weren't there, at least not without a Breakthrough).
I generally disliked the mechanics, really. I didn't run the system for making magical characters through anything, but it left the impression of being directed towards making magical player-characters, instead of making magical non-player characters. Seemed too balanced and mechanical, and not story and balance-as-opponent/ally oriented, to serve the storyguide well. I think that's a miss - the system should be designed to support the storyguide principally, with specific pathways and rules (like Ascendancy to the Hall of Heroes) for player characters.
Speaking of the connection to other works, I found it lacking. I don't think the book addressed, at all, how its rules relate to the various other rules provided for Immortal magi in TMRE. It mentioned Bjornaer Great Beasts, but not how their mechanics for Bjornaer initiates relate to those in Final Twilight - are they Transformed Animals? Is their progression now cut off from that of living Bjornaer in HoHMC? I think a small insert on Great Beasts and what is it they do (and where) could have been a very nice addition to RoPM.
I also disliked the power-boost. I didn't like that the main places in the Magic Realm had an aura of 10, yet the effective aura in a boundary was as high as 30. (I also didn't like the fact that the Twilight Void's aura wasn't mention - I think treating it as a +10 Form-oriented "boundary" is most appropriate, but it's an oversight.) I also didn't like the power boost associated with Art-Aligned Auras (I think a small bonus, perhaps during experimentation, would have conveyed the same flavor with less munchkinism). Directed vis, counting as two pawns in two separate Arts, was just weird - not a power boost, just weird.
Speaking of the Magic Realm - I found it just OK. Having regiones separated by the Twilight Void was fine, but felt more of a magic junkyard than a magic realm. I guess one could see the Twilight Void as the true magic realm (a point of view I found oddly lacking from the text), but its Form-aligned principalities make this explanation not work too well. I did like that the book allowed lots of leeway in interpreting what the Magic Realm is, and even vaguely remember an inside-joke about chambers, nails, and pots somewhere in there - I wonder if Michael de Vertil (sp.?) would be pleased.
I almost liked the three principles of magic. I have long thought of the Essential Nature idea as the defining one for Magic, but I think it was botched. RoPM says, for example, that to create a magical animal one should first think of how it is without magic, and this is its Essential Nature. Au contraire - I would have said that the thing's essential nature is what is revealed through Magic. Magic doesn't adorn your essential nature with bells and whistles - it reveals it in all its Might. In this way a brave person is not "transformed" to a Lion of Virtue; this is not a Transformation, but a Distillation, a Purification, a Revelation - shedding the dirt of mundanity to reveal the gem that lies underneath.
Likewise the second principle, that magic is ancient, is I think botched. I don't think it is conductive to say, as the book does, that the older a thing is the more magical it gets. Rather, it is that the most magical things are the most ancient ones - that Magic is often, though not always, in decline. I would have built the entire cosmology based on this single premise, identifying the Magic Realm as the timeless place where things are at their purest and most ancient forms, and the mundane realm as the domain of time, within which the perfections of Magic tend to erode. (Personally, I'd go further, defining three directions of erosion - the Infernal, which is towards nothingness, for God has not created Evil - evil is simply the Lack of good; the Faerie, which is the Unreal, the Imagination, the Dream, it is things-as-they-could-be, and therefore touches on things-as-most-perfect, which is Magic; and the Divine, which is things-as-they-cannot be, the Transcendent, pushing things not towards their Essential Nature but rather towards being better than themselves. But this is perhaps less suited to the official line.)
Finally, there is the principle of Occultism. I initially didn't like it, but seeing the mechanics of Acclimation drove the point home and it suddenly made lots of sense. Again, I blame the rhetoric. I think it could have been explained better. As I now understand it, the principle is that the most magical is most removed from the mundane; not that spreading word of the magical will make it less-so, which strikes me as a strange atavistic return to the dark days of the Realm of Reason.
And speaking of Acclimation - what are the rules for a character spending more than one season in the Magic Realm? Clearly in the Twilight Void there is no Acclimation, but what happens in cosms/whatever themselves? I was moved to imagine the Great Beasts of House Bjornaer strolling out of the Elysian Fields (?) to highly magical places through vestiges, spending months and years in the Twilight Void in between or choosing to Initiate a Bjornaer magus and stay longer in these more "mundane" places. I'm not sure the mechanics support such a state of affairs, however.
Vestiges, BTW, I find to be a grand idea. Very colorful and fun and interesting. But I very much dislike that they are partly immaterial, especially the strange idea that large ones are less so or that they are material only to beings aligned with the right Form. What benefit is there to any of this? Stepping into a grand magical tree to reach a forest in another realm is magical and grand; stepping into a half-substantive tree that is actually substantive to the nearby Treant-like creature is just weird.
I was a bit disappointed with RoPM. It had some great ideas in it, but I think it also botched the execution of some of them, which for me was a first in the spiffy ArM5 line. It's still very nice, and I'd recommend it to anyone in a heartbeat - but I feel a lot was missed.